1. Field of the Invention
The present invention relates to a cylinder head of a DOHC (Double Overhead Camshaft) type engine.
2. Description of the Background Art
A cylinder head of a DOHC engine is described in Japanese Patent No. 2584601. The cylinder head includes a tightening bolt for a cam holder, which is tightened in a direction at right angles to a mating surface of the cylinder head and the cam holder.
Providing a cylinder head with such a structure is unfavorable to maintenance performance because the tightening bolt overhangs an extension line of the plug hole. This is also unfavorable to making the space between the air intake and exhaust camshafts narrower and the cylinder head more compact. Moreover, this problem will be further exasperated if such a structure is adopted that the mating surface of the cam holder and the cylinder head is inclined with respect to the mating surface of the cylinder head and the cylinder.
FIG. 3 shows another structure in accordance with the background art. In FIG. 3, a cam holder mating surface c between a cylinder head a and a cam holder b is inclined with respect to a cylinder head mating surface d between the cylinder head a and the cylinder (not shown in FIG. 3). A head gasket e is located adjacent the cylinder head mating surface d.
A lower plug hole f is provided in the cylinder head a at a right angle to the cylinder head mating surface d. An ignition plug g is located inside the lower plug hole f. An electrode section of the ignition plug g reaches to a combustion chamber h.
An upper plug hole k communicates with the lower plug hole f. The upper plug hole k is provided in the cam holder b. The upper plug hole k is formed at a right angle to the cam holder mating surface c, so that the upper plug hole f is bent relative to the lower plug hole f.
An air intake side camshaft m and an exhaust side camshaft n are arranged on the cam holder mating surface c. The air intake side camshaft m and an exhaust side camshaft n are located on either side of the lower plug hole f and the ignition plug g. The air intake side camshaft m and an exhaust side camshaft n are held by cam holder tightening bolts p, fixing the cam holder b on the cylinder head a. The cam holder tightening bolts p are provided at right angles to the cam holder mating surface c.
Such a structure makes it difficult to make the cylinder head compact because the external shape of the cam holder b projects toward the inclined direction by the size of the head section of the bolt p. In addition, a part of the cam holder b surrounding the upper plug hole k overhangs an extension line of the lower plug hole f and enters the extension line of the lower plug hole f by a dimension D. This arrangement makes it difficult to install and remove the ignition plug g. Further, the overhang sets a limit on making the air intake side camshaft m and exhaust side camshaft n closer.